Biocompatible SWCNT Conductive Composites for Biomedical Applications

The efficiency of devices for biomedical applications, including tissue engineering and neuronal stimulation, heavily depends on their biocompatibility and performance level. Therefore, it is important to find adequate materials that meet the necessary requirements such as (i) being intrinsically compatible with biological systems, (ii) providing a sufficient electronic conductivity that promotes efficient signal transduction, (iii) having "soft" mechanical properties comparable to biological structures, and (iv) being degradable in physiological solution. We have developed organic conducting biocompatible single-walled carbon nanotubes (SWCNT) composites based on bovine serum albumin, carboxymethylcellulose, and acrylic polymer and investigated their properties, which are relevant for biomedical applications. This includes ζ-potential measurements, conductivity analyses, and SEM micrographs, the latter providing a local analysis of SWCNT distribution in the base material. We observed the development of the electrical conductivity of the SWCNT composites exposed to 1 mM KCl electrolyte for 40 days, representing a high stability of the samples. The conductivity of samples reaches 1300 S/m for 0.45 wt.% nanotubes. Moreover, we demonstrated the biocompatibility of the composites via cultivating fibroblast cell culture. Finally, we showed that composite coating results in the longer lifespan of cells on the surface. Overall, the SWCNT-based conductive composites might be a promising material for extended biomedical applications.

Development and in vitro validation of flexible intraretinal probes

The efforts to improve the treatment efficacy in blind patients with retinal degenerative diseases would greatly benefit from retinal activity feedback, which is lacking in current retinal implants. While the door for a bidirectional communication device that stimulates and records intraretinally has been opened by the recent use of silicon-based penetrating probes, the biological impact induced by the insertion of such rigid devices is still unknown. Here, we developed for the first time, flexible intraretinal probes and validated in vitro the acute biological insertion impact in mouse retinae compared to standard silicon-based probes. Our results show that probes based on flexible materials, such as polyimide and parylene-C, in combination with a narrow shank design 50 µm wide and 7 µm thick, and the use of insertion speeds as high as 187.5 µm/s will successfully penetrate the retina, reduce the footprint of the insertion to roughly 2 times the cross-section of the probe, and induce low dead cell counts, while keeping the vitality of the tissue and recording the neural activity at different depths.

Viral rhodopsins 1 are an unique family of light-gated cation channels

Phytoplankton is the base of the marine food chain as well as oxygen and carbon cycles and thus plays a global role in climate and ecology. Nucleocytoplasmic Large DNA Viruses that infect phytoplankton organisms and regulate the phytoplankton dynamics encompass genes of rhodopsins of two distinct families. Here, we present a functional and structural characterization of two proteins of viral rhodopsin group 1, OLPVR1 and VirChR1. Functional analysis of VirChR1 shows that it is a highly selective, Na⁺/K⁺-conducting channel and, in contrast to known cation channelrhodopsins, it is impermeable to Ca²⁺ ions. We show that, upon illumination, VirChR1 is able to drive neural firing. The 1.4 Å resolution structure of OLPVR1 reveals remarkable differences from the known channelrhodopsins and a unique ion-conducting pathway. Thus, viral rhodopsins 1 represent a unique, large group of light-gated channels (viral channelrhodopsins, VirChR1s). In nature, VirChR1s likely mediate phototaxis of algae enhancing the host anabolic processes to support virus reproduction, and therefore, might play a major role in global phytoplankton dynamics. Moreover, VirChR1s have unique potential for optogenetics as they lack possibly noxious Ca²⁺ permeability.

Nucleocytoplasmic Large DNA Viruses (NCLDV) that infect algae encode two distinct families of microbial rhodopsins. Here, the authors characterise two proteins form the viral rhodopsin group 1 OLPVR1 and VirChR1, present the 1.4 Å crystal structure of OLPVR1 and show that viral rhodopsins 1 are light-gated cation channels.

Surface Functionalization of Platinum Electrodes with APTES for Bioelectronic Applications

The interface between electronic components and biological objects plays a crucial role in the success of bioelectronic devices. Since the electronics typically include different elements such as an insulating substrate in combination with conducting electrodes, an important issue of bioelectronics involves tailoring and optimizing the interface for any envisioned applications. In this paper, we present a method for functionalizing insulating substrates (SiO₂) and metallic electrodes (Pt) simultaneously with a stable monolayer of organic molecules ((3-aminopropyl)triethoxysilane (APTES)). This monolayer is characterized by high molecule density, long-term stability, and positive surface net charge and most likely represents a self-assembled monolayer (SAM). It facilitates the conversion of biounfriendly Pt surfaces into biocompatible surfaces, which allows cell growth (neurons) on both functionalized components, SiO₂ and Pt, which is comparable to that of reference samples coated with poly-L-lysine (PLL). Moreover, the functionalization greatly improves the electronic cell-chip coupling, thereby enabling the recording of action potential signals of several millivolts at APTES-functionalized Pt electrodes.

Establishing a sensitive fluorescence-based quantification method for cyclic nucleotides

BACKGROUND

Approximately 40% of prescribed drugs exert their activity via GTP-binding protein-coupled receptors (GPCRs). Once activated, these receptors cause transient changes in the concentration of second messengers, e.g., cyclic adenosine 3',5'-monophosphate (cAMP). Specific and efficacious genetically encoded biosensors have been developed to monitor cAMP fluctuations with high spatial and temporal resolution in living cells or tissue. A well characterized biosensor for cAMP is the Förster resonance energy transfer (FRET)-based Epac1-camps protein. Pharmacological characterization of newly developed ligands acting at GPCRs often includes numerical quantification of the second messenger amount that was produced.

RESULTS

To quantify cellular cAMP concentrations, we bacterially over-expressed and purified Epac1-camps and applied the purified protein in a cell-free detection assay for cAMP in a multi-well format. We found that the biosensor can detect as little as 0.15 pmol of cAMP, and that the sensitivity is not impaired by non-physiological salt concentrations or pH values. Notably, the assay tolerated desiccation and storage of the protein without affecting Epac1-camps cyclic nucleotide sensitivity.

CONCLUSIONS

We found that determination cAMP in lysates obtained from cell assays or tissue samples by purified Epac1-camps is a robust, fast, and sensitive assay suitable for routine and high throughput analyses.

Electrochemical dual-aptamer biosensors based on nanostructured multielectrode arrays for the detection of neuronal biomarkers

Multielectrode arrays (MEAs) have been increasingly used for the development of biosensors due to their capability to record signals from multiple channels, fast mass transfer rates, and high spatial resolution. Alzheimer's disease (AD) is often associated with mitochondrial dysfunction, which is closely related to reduced levels of adenosine triphosphate (ATP). Therefore, simultaneous detection of ATP together with amyloid-β oligomers (AβO), a reliable biomarker for AD, can potentially advance the early detection of Alzheimer's disease. In this work, a dual-aptamer modified MEA chip was developed that consists of microelectrodes modified with electrodeposited 3D nanostructures (3D-GMEs). Electrodeposition methods, deposition potential, and deposition time were systematically altered and the active surface areas as well as the electrode morphologies were characterized by cyclic voltammetry and scanning electron microscopy. The nanostructured microelectrodes were sequentially modified with AβO and ATP specific aptamer receptors. To achieve the modification of different aptamer receptors at different 3D-GMEs of the same MEA chip, electrochemical cleaning was applied to individual 3D-GMEs. Ferrocene labels were attached to the aptamer receptors to enable amperometric signaling after target-aptamer binding. The developed aptasensor showed a linear detection range from 1 pM to 200 nM for the detection of AβO and from 0.01 nM to 1000 nM for the detection of ATP. Finally, ATP and AβO were detected simultaneously in the same analyte solution by the same sensor chip, which could support the early detection of AD, provide comprehensive information about the health status of the patient, and be helpful for pathological studies of neurodegenerative diseases.

Surface Plasmon-Enhanced Switching Kinetics of Molecular Photochromic Films on Gold Nanohole Arrays

Diarylethene molecules are discussed as possible optical switches, which can reversibly transition between completely conjugated (closed) and nonconjugated (open) forms with different electrical conductance and optical absorbance, by exposure to UV and visible light. However, in general the opening reaction exhibits much lower quantum yield than the closing process, hindering their usage in optoelectronic devices. To enhance the opening process, which is supported by visible light, we employ the plasmonic field enhancement of gold films perforated with nanoholes. We show that gold nanohole arrays reveal strong optical transmission in the visible range (∼60%) and pronounced enhancement of field intensities, resulting in around 50% faster switching kinetics of the molecular species in comparison with quartz substrates. The experimental UV-vis measurements are verified with finite-difference time-domain simulation that confirm the obtained results. Thus, we propose gold nanohole arrays as transparent and conductive plasmonic material that accelerates visible-light-triggered chemical reactions including molecular switching.

Polyethylene glycol-mediated blocking and monolayer morphology of an electrochemical aptasensor for malaria biomarker detection in human serum

Better approaches are critically needed for in situ point-of-care diagnostic biosensors that enable primary care physicians, or even individual patients, to directly analyze biological fluids without complicated sample pretreatments. Additional purification steps consume time, consume reagents, often require other equipment, and can introduce false-negative results. Biosensors have been modified with blocking molecules to reduce biofouling; however, the effectiveness relies on their chemical composition and morphology. Here, we used a polyethylene glycol film to suppress unspecific binding from human serum on an electrochemical malaria aptasensor. A detailed study of the variation of the chemical and morphological composition of the aptamer/polyethylene glycol mixed monolayer as a function of incubation time was conducted. Higher resistance to matrix biofouling was found for polyethylene glycol than for hydrophobic alkanethiol films. The best sensor performance was observed for intermediate polyethylene glycol immobilization times. With prolonged incubation, phase separation of aptamer, and polyethylene glycol molecules locally increased the aptamer density and thereby diminished the analyte binding capability. Remarkably, polyethylene glycols do not affect the aptasensor sensitivity but enhance the complex matrix tolerance, the dynamic range, and the limit of detection. Careful tuning of the blocking molecule immobilization is crucial to achieving high aptasensor performance and biofouling resistance.

Platinum substrate for surface plasmon microscopy at small angles

Platinum is reported as the main component of the substrate in surface plasmon microscopy of the metal-dielectric interface for small-angle measurements. In the absence of a narrow dip in the angular spectrum of platinum, the refractive index of the dielectric medium or the thickness of a deposited layer is proven deducible from the observed sharp peak, close to the critical angle. The sensitivities of refractive index and thickness measurements using platinum are compared with that of a gold surface plasmon resonance chip. Furthermore, the thickness of a structured layer of (3-Aminopropyl)triethoxysilane on the platinum substrate is measured to be 0.7 nm, demonstrating the high sensitivity of the technique.

Engineering Biocompatible Interfaces via Combinations of Oxide Films and Organic Self-Assembled Monolayers

In this paper, we demonstrate that cell adhesion and neuron maturation can be guided by patterned oxide surfaces functionalized with organic molecular layers. It is shown that the difference in the surface potential of various oxides (SiO₂, Ta₂O₅, TiO₂, and Al₂O₃) can be increased by functionalization with a silane, (3-aminopropyl)-triethoxysilane (APTES), which is deposited from the gas phase on the oxide. Furthermore, it seems that only physisorbed layers (no chemical binding) can be achieved for some oxides (Ta₂O₅ and TiO₂), whereas self-assembled monolayers (SAM) form on other oxides (SiO₂ and Al₂O₃). This does not only alter the surface potential but also affects the neuronal cell growth. The already high cell density on SiO₂ is increased further by the chemically bound APTES SAM, whereas the already low cell density on Ta₂O₅ is even further reduced by the physisorbed APTES layer. As a result, the cell density is ∼8 times greater on SiO₂ compared to Ta₂O₅, both coated with APTES. Furthermore, neurons form the typical networks on SiO₂, whereas they tend to cluster to form neurospheres on Ta₂O₅. Using lithographically patterned Ta₂O₅ layers on SiO₂ substrates functionalized with APTES, the guided growth can be transferred to complex patterns. Cell cultures and molecular layers can easily be removed, and the cell experiment can be repeated after functionalization of the patterned oxide surface with APTES. Thus, the combination of APTES-functionalized patterned oxides might offer a promising way of achieving guided neuronal growth on robust and reusable substrates.

Correction: Tantalum(v) 1,3-propanediolate β-diketonate solution as a precursor to sol–gel derived, metal oxide thin films

Correction for ‘Tantalum(v) 1,3-propanediolate β-diketonate solution as a precursor to sol–gel derived, metal oxide thin films’ by Christopher Beale et al., RSC Adv., 2020, 10, 13737–13748, DOI: 10.1039/D0RA02558E.

Tantalum(v) 1,3-propanediolate β-diketonate solution as a precursor to sol–gel derived, metal oxide thin films

Tantalum oxide is ubiquitous in everyday life, from capacitors in electronics to ion conductors for electrochromic windows and electrochemical storage devices. Investigations into sol–gel deposition of tantalum oxide, and its sister niobium oxide, has accelerated since the 1980s and continues to this day. The aim of this study is to synthesize a near UV sensitive, air stable, and low toxicity tantalum sol–gel precursor solution for metal oxide thin films – these attributes promise to reduce manufacturing costs and allow for facile mass production. By utilizing 1D and 2D nuclear magnetic resonance, this study shows that by removing ethanol from the precursor solution at a relatively low temperature and pressure, decomposition of the photosensitive complex can be minimized while obtaining a precursor solution with sufficient stability for storage and processing in the atmosphere. The solution described herein is further modified for inkjet printing, where multiple material characterization techniques demonstrate that the solution can be utilized in low temperature, photochemical solution deposition of tantalum oxide, which is likely amorphous. Tested substrates include amorphous silica, crystalline silicon wafer, and gold/titanium/PET foil. The hope is that these results may spark future investigations into electronic, optical, and biomedical device fabrication with tantalum oxide, and potentially niobium oxide, based films using the proposed synthesis method.

Synthesis of tantalum(v) 1,3-propanediolate β-diketonate solution and use in photochemical solution deposition to form tantalum oxide films.

MEA Recordings and Cell-Substrate Investigations with Plasmonic and Transparent, Tunable Holey Gold

Microelectrode arrays are widely used in different fields such as neurobiology or biomedicine to read out electrical signals from cells or biomolecules. One way to improve microelectrode applications is the development of novel electrode materials with enhanced or additional functionality. In this study, we fabricated macroelectrodes and microelectrode arrays containing gold penetrated by nanohole arrays as a conductive layer. We used this holey gold to optically excite surface plasmon polaritons, which lead to a strong increase in transparency, an effect that is further enhanced by the plasmon's interaction with cell culture medium. By varying the nanohole diameter in finite-difference time domain simulations, we demonstrate that the transmission can be increased to above 70% with its peak at a wavelength depending on the holey gold's lattice constant. Further, we demonstrate that the novel transparent microelectrode arrays are as suitable for recording cellular electrical activity as standard devices. Moreover, we prove using spectral measurements and finite-difference time domain simulations that plasmonically induced transmission peaks of holey gold red-shift upon sensing medium or cells in close vicinity (<30 nm) to the substrate. Thus, we establish plasmonic and transparent holey gold as a tunable material suitable for cellular electrical recordings and biosensing applications.

Amperometric Aptasensor for Amyloid-β Oligomer Detection by Optimized Stem-Loop Structures with an Adjustable Detection Range

Amyloid-β oligomers (AβO) have become representative biomarkers for early diagnosis of Alzheimer's disease. Here, we report on an aptasensor based on stem-loop probes for sensitive and specific detection of AβO by an amperometric transducer principle using alternating current voltammetry (ACV). Stem-loop probes with redox-active moieties are immobilized on a gold substrate as a receptor element. The signal transduction mechanism relies on redox ferrocene (Fc) reporting via charge transfer on a molecular recognition event involving a conformational change of the molecular beacon. The stem-loop structures were optimized by considering the aptamers' stem length, spacer, and different ferrocene terminals. In addition, the sensor assembly and signal recording including aptamer concentration and ACV frequency dependence are discussed. Using the optimized stem-loop probe (B-3' Fc), the aptasensor showed a decrease of the Fc peak current induced by AβO binding within the broad concentration range spanning 6 orders of magnitude. Furthermore, the detection limit of the sensor can be further decreased by optimizing the ACV frequency, however at the cost of a narrowed detection range. In this work, a label-free electrochemical aptasensor is demonstrated, which facilitates the quantification of the concentration of AβO with high selectivity and subpicomolar sensitivity, which may be conducive to improving the diagnosis and pharmacology studies of Alzheimer's disease.

Improvements of Microcontact Printing for Micropatterned Cell Growth by Contrast Enhancement

Patterned neuronal cell cultures are important tools for investigating neuronal signal integration, network function, and cell-substrate interactions. Because of the variable nature of neuronal cells, the widely used coating method of microcontact printing is in constant need of improvements and adaptations depending on the pattern, cell type, and coating solutions available for a certain experimental system. In this work, we report on three approaches to modify microcontact printing on borosilicate glass surfaces, which we evaluate with contact angle measurements and by determining the quality of patterned neuronal growth. Although background toxification with manganese salt does not result in the desired pattern enhancement, a simple heat treatment of the glass substrates leads to improved background hydrophobicity and therefore neuronal patterning. Thirdly, we extended a microcontact printing process based on covalently linking the glass surface and the coating molecule via an epoxysilane. This extension is an additional hydrophobization step with dodecylamine. We demonstrate that shelf life of the silanized glass is at least 22 weeks, leading to consistently reliable neuronal patterning by microcontact printing. Thus, we compared three practical additions to microcontact printing, two of which can easily be implemented into a workflow for the investigation of patterned neuronal networks.

Silicon Nanowires Field Effect Transistors: A Comparative Sensing Performance between Electrical Impedance and Potentiometric Measurement Paradigms

Potentiometric sensors based on silicon nanowire field effect transistors (SiNW FETs) typically display exquisite sensitivities, but their bioanalytical implementation is limited due to the need for stringent measurement conditions and high-precision readout units. An alternative operation principle where SiNW FETs are operated in a frequency-domain electrical impedimetric approach is promising. However, to date only limited data is available in regard to the sensing performance and translational relevance of this novel approach in comparison to the standard charge detection paradigm. We demonstrate the feasibility of conducting electrical impedimetric FET measurements with a portable unit for the ultrasensitive detection of cancer biomarkers in biospecimens. Compared to standard potentiometric measurements, electrical impedimetric FET measurements yielded significant improvements in biosensing performances, including the limit of detection, sensing resolution, and dynamic range.

Amplification of aptamer sensor signals by four orders of magnitude via interdigitated organic electrochemical transistors

Electrochemical aptamer receptor/transducer systems are key elements of emerging E-AB sensors (aptasensor) used for the detection of various kinds of targets. However, the performance of these amperometric sensors is often limited by the low density of receptors attached to the sensor surface and high background signals. In the present work, interdigitated organic electrochemical transistors (iOECT) were used as a transducer to enhance the sensitivity and dynamic detection range of aptasensors. Therefore, the electrode of an amperometric sensor was utilized as gate electrode to operate the iOECT. This device was used to detect the low weight target molecule adenosine triphosphate (ATP), a common biomarker, which plays an important role for cardiovascular, neurodegenerative, and immune deficiency diseases. The novel aptasensor can selectively detect ATP with ultrahigh sensitivity down to the concentration of 10 pM, which is four orders of magnitude lower than the detection limit of the same aptasensor using an amperometric transducer principle (limit-of-detection of 106 nM) and most other previously reported electrochemical sensors. Furthermore, sensor regeneration was demonstrated, which facilitates reusability of OECT aptasensors. The small device size in combination with high transconductances paves the way for the development of highly sensitive integrated micro-biosensors for point-of-care applications.

Sensor Configuration and Algorithms for Power-Line Interference Suppression in Low Field Nuclear Magnetic Resonance

Low field (LF) nuclear magnetic resonance (NMR) shows potential advantages to study pure heteronuclear J-coupling and observe the fine structure of matter. Power-line harmonics interferences and fixed-frequency noise peaks might introduce discrete noise peaks into the LF-NMR spectrum in an open environment or in a conductively shielded room, which might disturb J-coupling spectra of matter recorded at LF. In this paper, we describe a multi-channel sensor configuration of superconducting quantum interference devices, and measure the multiple peaks of the 2,2,2-trifluoroethanol J-coupling spectrum. For the case of low signal to noise ratio (SNR) < 1, we suggest two noise suppression algorithms using discrete wavelet analysis (DWA), combined with either least squares method (LSM) or gradient descent (GD). The de-noising methods are based on spatial correlation of the interferences among the superconducting sensors, and are experimentally demonstrated. The DWA-LSM algorithm shows a significant effect in the noise reduction and recovers SNR > 1 for most of the signal peaks. The DWA-GD algorithm improves the SNR further, but takes more computational time. Depending on whether the accuracy or the speed of the de-noising process is more important in LF-NMR applications, the choice of algorithm should be made.

Composite Lipid Bilayers from Cell Membrane Extracts and Artificial Mixes as a Cell Culture Platform

An artificial lipid bilayer is the closest possible model for the cell membrane. Despite that, current methods of lipid bilayer assembly and functionalization do not provide a satisfactory mimic of the cell-cell contact due to the inability to recreate an asymmetrical multicomponent system. In the current work, a method to produce an integrated solid-supported lipid bilayer combining natural extracts from cell membranes and artificially made lipid vesicles is proposed. This simple method allows delivery of transmembrane proteins and components of the extracellular matrix into the substrate. Biocompatibility of the composite natural/artificial lipid bilayers is evaluated by their interactions with the cardiomyocyte-like HL-1 cell line. Compared with fully artificial mixes, composite lipid bilayers allow cells to adhere and develop a morphologically more normal cytoskeleton.

Multiplex Detection of Different Magnetic Beads Using Frequency Scanning in Magnetic Frequency Mixing Technique

In modern bioanalytical methods, it is often desired to detect several targets in one sample within one measurement. Immunological methods including those that use superparamagnetic beads are an important group of techniques for these applications. The goal of this work is to investigate the feasibility of simultaneously detecting different superparamagnetic beads acting as markers using the magnetic frequency mixing technique. The frequency of the magnetic excitation field is scanned while the lower driving frequency is kept constant. Due to the particles’ nonlinear magnetization, mixing frequencies are generated. To record their amplitude and phase information, a direct digitization of the pickup-coil’s signal with subsequent Fast Fourier Transformation is performed. By synchronizing both magnetic fields, a stable phase information is gained. In this research, it is shown that the amplitude of the dominant mixing component is proportional to the amount of superparamagnetic beads inside a sample. Additionally, it is shown that the phase does not show this behaviour. Excitation frequency scans of different bead types were performed, showing different phases, without correlation to their diverse amplitudes. Two commercially available beads were selected and a determination of their amount in a mixture is performed as a demonstration for multiplex measurements.